An optical mode converter performs mode conversion to manipulate a size of an optical signal. For example, a silicon-on-insulator (SOI) waveguide input/output port may a have a size of about 2-4 micrometers (um) and a single mode fiber coupling light to the SOI waveguide may have a size of about 8-9 um. An optical mode converter may couple the single mode fiber to the SOI waveguide to provide mode conversion (size conversion). The optical mode converter may be an open path (e.g., include an air gap) such that the optical mode converter is packaged in a hermetically sealed package to provide suitable performance of the optical mode converter, which may increase a complexity and cost of production of the optical mode converter.